1. Field of the Invention
The present invention relates to container liners used for the transport of compactable, cohesive powders. More specifically the present invention relates to a device for fluidising powdered material contained within a container liner within a shipping container.
2. Background Art
There are many dry materials in fine particulate form which readily compact and bind together, showing a characteristically high angle of repose and difficult flow properties. Such powders commonly cause great difficulty on discharge from a shipping container, as they will not flow freely when the container is tipped, even at angles of 45° or more.
In order to facilitate the discharge of such materials from shipping and other storage containers it is known to fluidise the material by causing air to pass through it.
One known device for achieving fluidisation of powdered materials within a container liner comprises a fluidising mat which is placed within and at the bottom of the container liner prior to filling. The mat comprises two layers which are sealed along their edges and the uppermost of these two layers has micro-perforations in it Air under pressure is introduced between the two layers and passes through the micro-perforations in the upper layer to cause fluidisation of the powdered material contained within the liner.
Although this known device has succeeded to some degree in facilitating the discharge of fine particulate compactable powders from storage containers, there are a number of disadvantages which have prevented its widespread use. These disadvantages include:                a) The fact that the two flexible layers of the mat lie flat one on top of the other during loading, storage and transport. Given the weight of the cargo that lies on the mat in use, there is considerable resistance to the penetration of the injected gas between the layers, and hence to its even distribution under the cargo.        b) Many of the mats are divided into discrete sections or pockets in order to facilitate better control of the distribution of the injected gas. However, the ridges created between the pockets commonly trap material making discharge difficult. Moreover, the separation of the mat into discrete pockets also requires the use of a complex manifold to distribute the air under pressure to each of these pockets.        c) Material is commonly trapped between the edge of the fluidising mat and the container walls from where it is difficult to discharge. Attempts to overcome this problem by extending the fluidising mat up the sides of the container have met with limited success due to creasing problems and the higher initial cost of the mat.        d) When the discrete pockets of the fluidising mat are inflated they have a tendency to balloon. As the width of the pockets reduces the fluidising mat tends to pull away from the side walls of the container. This results in material being trapped down the sides of the fluidising mat from where it is difficult to discharge. To solve this problem it has been proposed to secure the fluidising mat along its edges to the liner and thereby maintain it at its full width, but this solution has proved complicated, expensive and not very effective.        e) The pervasive dusting which follows fluidisation by the high pressure air commonly used in this type of mat generally requires the provision of filtration equipment to reduce the egress of dust into the atmosphere.        
WO-93/04954 discloses a fluidising mat which seeks to overcome at least some of these disadvantages. The mat is of rigid construction and comprises an upper non-flexible sheet and a lower non-flexible gas impermeable sheet held in spaced apart superimposed relationship by a plurality of rigid ribs which extend longitudinally between the upper and lower planar sheets to form an array of separate channels that extend substantially the full area of the mat. These channels are open at one end and closed at the other. Air is fed to each of the channels by a gas manifold which extends the full width of the mat to take in all the open ends of the channels. The upper planar sheet and the upper portion of the gas manifold have a plurality of micro-perforations therein.
The rigid structure of this fluidising mat has several advantages. Since the upper and lower layers of the mat are permanently held apart by the rigid ribs, the injected air is freely distributed and flows to all parts of the mat. Furthermore the gap between the upper and lower layers provides an effective plenum chamber which in turn ensures an even pressure distribution over the surface of the mat.
The rigid construction of this mat also prevents any ballooning of the mat with the resultant retraction from the walls of the container. Furthermore, it can eliminate the ridges between the sections of a flexible mat, since all parts of the mat can be of the same thickness.
A further advantage of this fluidising mat is that there is no need to inject high pressure air into the mat to lift the cargo and separate the upper and lower layers of the mat thereby allowing distribution of the air within the mat.
Although this fluidising mat provides means for facilitating the discharge of fine particulate compactable powders from storage containers, there are still a number of practical disadvantages which have limited its commercial use. These are:                a) The rigidity of the fluidising mat often necessitates that it be fitted at the time of fitting the liner. This in turn requires that fitters enter the liner through the hatches of the container. However, the possibility of ingress of contamination into the liner does in many cases make this totally unacceptable.        b) Commercial exploitation of the rigid mat has been further hampered by the high cost of this type of mat.        c) The size of the liner and fluidising mat together makes transport difficult, since the rigid mat cannot be folded down in size like the liner.        d) The design of the mat with separate longitudinally extending channels requires the use of a relatively complex manifold to connect the open end of each channel to the air supply.        